The invention relates to a blade shape for a Francis hydraulic turbine and particularly to the inlet angle and thickness of the blade.
The Francis turbine is a commonly used water turbine. Francis turbines are suitable for operation with pressure heads of, for example, ten (10) meters to six hundred and fifty (650) meters. These turbines are often used to drive generators to produce electrical power, such as in the range of 10 to 750 megawatts. Examples of Francis turbines are shown in U.S. Pat. Nos. 7,198,470; 7,195,460; 7,195,459; 7,128,534 and 6,135,716. The reference diameter of a Francis turbine, e.g., the runner diameter, may be 1 to 10 meters. The turbine may operate at rotational speeds of 80 to 1000 revolutions per minute (rpm). Large and mid-sized Francis turbines tend to rotate about a vertical shaft, whereas smaller turbines rotate about a horizontal shaft. This general description of a conventional Francis turbine is to provide a context for the blade shape disclosure herein and not to limit the scope of the invention.
A runner is the portion of a Francis turbine which includes the turbine blades. A new runner may be installed when rehabilitating a Francis turbine. The dimensions of the new runner are constrained by the dimensions of the chamber for the existing runner. The new runner is designed to fit into the housing. Due to the constraints of the existing housing, the design of the new runner may not be optimal to provide maximum peak efficiency and cavitation behavior for the turbine. Generally, the new runner is constrained to be more compact, e.g., have a smaller runner inlet diameter, than would be the optimal. Because the runner is compact, issues arise with respect to the efficiency of the turbine and its performance such as with respect to cavitation. These issues tend to be of a particular concern where the runner is to be used in a Francis turbine operating under a large pressure head, such as greater than 100 meters.
For a given pressure head, number of blades and fixed blade outlet elevation relative to distributor centerline, compact inlet diameters should increase the global blade loading. The blade inlet can therefore reach critical low pressure levels, resulting in either pressure or suction side cavitation, depending on operating head.